It's a BBY engine. I opened the oil cap with the engine running today to check for fumes and blow-by. The moment I did this idle became really rough and the engine started to stall. I closed the cap everything went back to normal. Did this a couple of times and always got the same result. MIL was there on the dashboard, too but haven't had the time to read error codes yet.

 

Is this normal for these engines? I cannot really figure out why that would be happening.

Yes, it's normal, it happens because the engine sucks fresh air in through the oil cap into the intake, this causes the mixture to lean off and the engine stalls.

Well that's the first engine design of this kind I encounter.

 

How does it suck fresh air in the intake through the oil cap? There is a ventilation hose connecting the valve cover with the intake but it ends way before the throttle body. So, it is the same as outside air to the engine.

The Positive Crankcase Ventilation system connects crankcase to intake via a valve on the back of the block. The outlet of the valve connects just below the throttle body into the inlet manifold. (A convoluted plastic hose)

 

The valve cover connection is a very narrow bore so only lets a small amount of air in relative to the oil cap opening.

Edited 19 October, 20205 yr by Wino

I've seen this problem occur over a few decades of overhead cam engines. It's not really a good idea to remove the oil cap with the engine running as apart from this problem, it can cause oil under crankcase pressure to splatter over the top of the engine and if you are too close without eye/face protection get a faceful/eyeful of hot oil under pressure.

^ There is no problem.

Crankcase pressure is slightly below atmospheric when the PCV system is working correctly.